First mammal

[TwinCrier]

In the process of evolution at what point, or what trait caused the creatures of Earth at that time to become mammals?

 

[Gold Dragon]

In the process of evolution at what point, or what trait caused the creatures of Earth at that time to become mammals?

One thing to keep in mind is that the term mammal (or any other taxonomic divider) is really an arbitrary collection of similar traits that have been observed in nature. As with most of these dividing lines, there are always exceptions where some of those traits are present in an animal but not all of them and the determination become fuzzy.

Wikipedia - Mammal

The mammals are the class of vertebrate animals primarily characterized by the presence of mammary glands in the female which produce milk for the nourishment of young; the presence of hair or fur; and which have endothermic or "warm blooded" bodies. The brain regulates endothermic and circulatory systems, including a four-chambered heart. Mammals embrace more than 5,000 genera, distributed in 425 families and up to 46 orders (the number depends on the classification scheme adopted).

Phylogenetically, the Mammalia are defined as the last common ancestor of monotremes (e.g., echidnas) and therian mammals (e.g., hedgehogs), and all of this last common ancestor's descendants.

How Stuff Works has a section on the evolution of mammals. I haven't read it but it may help.

 

[gluadys]

In the process of evolution at what point, or what trait caused the creatures of Earth at that time to become mammals?

There are many traits that distinguish mammals from their synapsid reptilian ancestors. A good listing of them is found here:

http://www.talkorigins.org/faqs/faq-transitional/part1b.html

Below the listing are descriptions of actual fossils. You can see how different characteristics of skull, teeth and jaw bones (and sometimes other features) changed over time.

A particularly interesting characteristic to watch is the jaw hinge. In reptiles the jaw is hinged at a different interface than in mammals. In mammals the bones which are used as a jaw hinge in reptiles end up as bones in the middle ear! And the jaw is hinged by a different pair of bones.

But how does a transitional species eat as it changes from one hinge to the other? Look for species in this list which have TWO jaw hinges.

Some major differences between reptiles and mammals are not easily preserved in fossils. When did animals change from a three-chambered to a four-chambered heart? When did they start replacing scales with hair? When did they start maintaining a constant body temperature? When did they begin suckling their young?

These are questions that are not easily answered from a skeleton.

The oldest mammal in this list is Adelobasileus cromptoni from the Triassic (time of early dinosaurs).

 

[TwinCrier]

So would it be possible for animals to have at one time posses traits of both mammals and reptiles or birds as they would be in transition? In most evolution books and PBS programs they refer to the time "mammals first appeared" so would that be the beginning or end of their transition?

 

[gluadys]

So would it be possible for animals to have at one time posses traits of both mammals and reptiles or birds as they would be in transition? In most evolution books and PBS programs they refer to the time "mammals first appeared" so would that be the beginning or end of their transition?

Don't bring birds into it. Mammals are descendants of therapsid reptiles. Birds are descendants of therapod dinosaurs. Completely different branches.

You would have to check out with PBS exactly what they mean by "the time mammals first appeared". They probably mean the time of the first fossil species that are classified as mammals.

But, as Gold Dragon said, there is actually a rather fuzzy line. There are many fossils classified as reptiles which have some characteristics of mammals and there are fossils classified as mammals which retain some of the features of reptiles.

So, the answer to your basic question is "yes, it is possible for a species of animal to have a mixture of traits of both reptiles and mammals". That list I linked you to gives many examples of this.

 

[TwinCrier]

But the current theory is that modern day birds evolved from dinosaurs. And didn't we all evolve from single celled organisms anyway? Are there animals alive today that have mixture of two species, both reptile and mammal, or did these creatures become extinct as their ancestors evolved?

 

[Gold Dragon]

Are there animals alive today that have mixture of two species, both reptile and mammal, or did these creatures become extinct as their ancestors evolved?

I found a better link for you.

Ancestors of Mammals

Humans are mammals, the most successful taxonomic class of organisms to colonise the Earth. The word mammal derives from the Latin meaning of breast, "mamma", where breasts are a consistent trait among mammals in mothers feeding for feeding their young. Coincidentally, the more scientific name for the breast is the mammary gland, which further illustrates the point.

Mammals are a diverse group of organisms, where the majority of them develop their offspring within the uterus of the mother, though exceptions are noted. For example, monotremes lay eggs, like their common ancestors the reptiles and birds.

To further diverse, over time mammals have diversified into the placentals and the marsupials. But before we get into that, first look at the ancestors of the mammals to get a better understanding of how the mammals became dominant in the first place, in accordance with natural selection and geological events.

Ancestors of Mammalia
The taxonomic class Mammalia is within the Vertebrata phylum, which elementarily suggests that the direct ancestors of mammals were vertebrates. This is true of course, as it would have allowed taxonomists to order the species in light of this.

Over three hundred million years ago, when life was beginning to conquer dry land, reptiles had adapted from their ancestors to live on the land, and acquire an ecological niche that otherwise had no competition.

It is believed that a niche of reptiles deemed the paramammals, which have sufficient distinctions between both reptiles and mammals, to suggest that mammals indeed evolved from reptiles.

Circumstantial Change
Although some reptiles were beginning to possess mammal-like features, it was not for another 50 million years that the first distinctive differences were being noticed in species. Land animals were continuing to diversify and occupy new ecological niches and move away from competitive environments. Herbivores soon diversified from the reptiles, while dog-like species were becoming dominant as a competitor to the more reptile-like creatures.

These dog like creatures were beginning to diversify in the land environment, and become a true competitor for land resources, unlike the more water-dependant reptiles. Characteristic changes like cold to warm-blooded, prolonged front teeth, fur and mammary glands helped taxonomists note the difference over time from the transition from reptiles to more mammal like creatures

 

[gluadys]

But the current theory is that modern day birds evolved from dinosaurs. And didn't we all evolve from single celled organisms anyway? Are there animals alive today that have mixture of two species, both reptile and mammal, or did these creatures become extinct as their ancestors evolved?

Mammals separated from reptiles a long time ago. Since they would likely outcompete their ancestors, yes, the ancestors apparently became extinct. The closest we have to a living intermediate are the three species of egg-laying mammals.

Yes, birds evolved from dinosaurs, in fact, not from dinosaurs generally, but from one specific group of dinosaurs. Mammals did not evolve from dinosaurs but from a different group of reptiles entirely.

It is likely that the closest unicellular relative to animals were the choanoflagellates

Sponges use choanoflagellate-type cells in their body structure, and sponges are believed to be the earliest true animals, from which all other animals branched off.

Look at the
choanocyte cells in this sponge and see the similarity to the independent choanoflagellates.

Be sure also to check out the Proterospongia which show an intermediate step between single-cells and multi-cellular organisms.